Servers respond to client GData requests with feeds that include lists of entries.
Each feed and entry is composed of elements. Elements represent either standard Atom XML elements, or custom-defined GData elements.
Feeds, entries, and elements are derived from GDataObject, the base class that implements XML parsing and generation.
おおよそのイメージはこんな感じなのだろうか。
参照：
gdata-objectivec-client -> Wiki -> GDataObjeCIntroduction -> Google Data APIs Basics
